1. Field of the Invention
The present invention relates to a developing device which carries out developing by a positively charged single-component developing agent transferring to an image carrier in an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus, such as an electronic copying machine, a developing device is known for developing an electrostatic latent image formed on a photosensitive body, using a single-component developing agent, as disclosed in IEEE/IAS, Conf. Record, 1985, P. 1485 (Hosoya et al.).
This device carries out developing in the following manner. A developing roller, which is a nickel plated aluminum roller, is rotated. A single-component toner, such as a single-component developing agent, is attracted to the surface of the developing roller by triboelectric charges between a stainless steel coating blade and single-component toner particles, and between the developing roller and the single-component toner particles. Then, the single-component toner particles on the surface of the developing roller are caused to adhere electrostatically to a photosensitive body at a developing station facing the photosensitive body. In such a device, when the photosensitive body includes an organic photoconductor (hereafter OPC), the OPC generally has a composition in which an electric charge generating diazo-type dye layer of about 1 .mu.m in thickness and a hydrazone dispersion-type electric charge transport layer of about 20 .mu.m in thickness are multilayered on a substrate. Therefore, this OPC is negatively charged, and for this reason the single-component developing agent used in this kind of photosensitive body must be positively charged.
However, a single-component toner is normally composed of carbon, silica or resin. Such a toner is difficult to charge positively by friction with metal, and most often the toner becomes negatively charged. For this reason, an agitating agent is often used to assist in the positive charging of a single-component toner, although it does not contribute to transport. The agitating agent functions in a manner similar to the carrier particles in a two-component developing agent. The agitating agent is made of iron particles coated with polytetrafluoroethylene. However, even with this kind of method, a sufficient triboelectric charge cannot be obtained, because there is less chance of friction between the single-component toner and the agitating agent. This leads to poor quality developing with randomness of image density and fogging of the image to randomness of the charge. Furthermore, there is the problem of soiling in the vicinity of the developing device because the single-component toner scatters easily due to insufficient adherence of the single-component toner with the developing roller.